Today, magnetic disk drives, and in particular hard disk drives, are essential peripheral devices as external storage devices for computers. In the field of personal computers, those with a small size and a large capacity are widely used.
A magnetic disk drive is in general a component integrally incorporating a plurality of magnetic disks, a disk rotation mechanism, a recording/reproduction head, and a head positioning servo mechanism. It uses the disk rotation mechanism to rotate the plurality of magnetic disks at a high speed, and moves the recording/reproduction head in the radial direction above the disk to reach a target track for recording or reproduction.
The head is provided at the tip of a head arm that is swung by an actuator around a rotation shaft. The actuator is controlled by a servo system to perform head seeking and target track following operations. A recording-reproduction-separated head that is a combination of an MR (magnetoresistive effect) head for reproduction and a thin film inductive head for recording is used as the recording/reproduction head.
Head control for such a magnetic disk drive generally has speed control and position follow-up modes. In the speed control mode, a speed profile (reference speed pattern) is generated according to the difference between a position of the head and the target position. A drive current in proportion to the difference between the speed profile and head speed is supplied to the actuator to match the head speed with the speed profile, allowing the head to move to the target track at a high speed. In the position follow-up mode, a control signal in proportion to the difference between the position of the head and the target position, and a control signal in proportion to the head speed are used to drive the actuator to control the head so that it will not deviate from the target track.
Such a proportional gain set for each head control mode is determined by the design values of the actuator. However, since dynamic parameters vary from one actuator and another, the gain is eventually adjusted and determined before shipment. For the position follow-up mode, an automatic gain controller has been proposed to maintain the loop gain constant by measuring the loop gain for the entire closed loop (Japanese patent unexamined publication No. 4-219801).
In addition, the disk drive requires a servo device to suppress the vibration of the head caused by the resonance of a head arm, a carriage, disks, etc.
In a disk drive employing an observer in a control system, however, if actuator parameters vary due to temperature changes or the characteristics of each actuator, attempts to compensate the variations with the automatic adjustment of the loop gain do not result in sufficient compensation for the servo characteristics because these parameters are built into in the observer.
In addition, if parameter variations due to the characteristics of the actuator are eventually adjusted before shipment, it is difficult to compensate variations due to secular changes.
With the automatic loop gain control device (the unexamined publication No. 4-219801), gain adjustment for the observer is difficult because loop gain adjustment depends solely upon output gain adjustment. Particularly, varying actuator parameters affect the speed profile following characteristics of the head in speed control mode and the automatic loop gain control device cannot eliminate this effect.
Furthermore, a disk drive that digitally controls the head is subject to alias noise as well as lost bits. To reduce the effect of alias noise, a detected positional error signal is conventionally used as an error control signal after it is caused to pass through an anti-alias filter or a lowpass filter. This prevents the generation of positional error signals for control if the head moves at a high speed and the high-frequency components in the positional error signals increase.
It is an object of the present invention to provide a control system that can stabilize the servo characteristics of a disk drive even when there are variations and secular changes in parameters for an actuator that moves a head.
It is another object of this invention to provide a control system that can detect positional error signals well to precisely control head positioning even when the head is moving at a high speed, such as in access operation.